There are four classes of dehydratases:
1. Dehydratases acting on 3-hydroxyacyl-CoA esters (no cofactors) 2. Dehydratases acting on 2-hydroxyacyl-CoA esters (radical reaction, [4Fe-4S] cluster containing) 3. Dehydratases acting on 4-hydroxyacyl-CoA esters ([4Fe-4S] and FAD containing) 4. Dehydratases containing an [4Fe-4S] cluster as active site (aconitase, fumarase, serine dehydratase)